The High Cost of “Close Enough”: Understanding Tolerances in Precision CNC Machining

The High Cost of “Close Enough”: Understanding Tolerances in Precision CNC Machining

In the world of precision manufacturing, there is a fine line between a masterpiece of engineering and a costly over-specification. At Accura CNC Supply, we often see designs that call for extremely tight tolerances—sometimes far beyond what the part’s actual function requires.

While our Haas and Hardinge equipment can hit incredible levels of accuracy, choosing the right tolerance is one of the most significant factors in determining your project’s lead time and final cost.

What are Tolerances, Anyway?

In CNC machining, a tolerance is the acceptable range of variation in the dimensions of a part. No machine can produce a part to an infinite degree of perfection every single time. Therefore, engineers specify a "window" (e.g., +/- 0.005”) within which the part will still function correctly.

Why Tighter Isn’t Always Better

It’s tempting to specify the tightest tolerance possible to ensure "quality." However, moving from a standard tolerance (+/- 0.005”) to a precision tolerance (+/- 0.0005”) can exponentially increase costs for several reasons:

• Increased Cycle Time: To achieve extreme precision, machines must run at slower speeds with lighter "finish passes."
• Specialized Tooling: Tight tolerances often require high-end carbide tooling or specialized boring bars that wear out faster.
• Inspection Rigor: A part with standard tolerances might be checked with calipers; a high-precision part requires CMM (Coordinate Measuring Machine) inspection and climate-controlled environments.
• Scrap Rates: The narrower the "window" of success, the higher the risk of a part falling outside of it, leading to higher material waste.

Finding the Sweet Spot: The Accura Approach

When we review a client’s RFQ, our goal is to ensure the part performs flawlessly while remaining cost-effective. Here are three tips for optimizing your designs:

• Identify Critical vs. Non-Critical Dimensions: Only apply tight tolerances where the part interacts with other components (like a bearing fit or a threaded hole). For the outer casing or aesthetic surfaces, a looser tolerance saves money.
• Consider the Material: Machining a +/- 0.001” tolerance in 6061 Aluminum is much easier than doing so in 304 Stainless Steel or high-temp alloys, which tend to shift and warp during the process.
• Use Standard Holes: Designing around standard drill bit sizes rather than custom-reamed holes can significantly reduce tool change time.

Why Accura CNC Supply?

Precision is our middle name (literally), but we believe in Smart Precision. By utilizing our fleet of Haas high-speed mills and Hardinge turning centers, we provide the repeatability you need for high-stakes industries like aerospace, medical, and automotive.

Our team doesn't just cut metal; we review your prints to ensure that your tolerances are working for you, not against your budget.

Get Started Today

Browse our equipment or contact us for a free consultation. Email info@accuracncsupply.com — we respond within 24 hours.